1. Field of the Invention
The present invention relates in general to hydraulic cylinders with actuating rod stroke sensors and, more particularly, to an improvement in a structure for mounting the actuating rod stroke sensor to the hydraulic cylinder. The actuating rod is provided with a magnetically transformed part so as to sense the piston's displacement during the reciprocating motions of the actuating rod.
2. Description of the Prior Art
As well known to those skilled in the art, construction equipment performs its operations under coarse operating conditions while generating considerable amounts of noise and vibrations, thereby being noted for causing accidents to surrounding workers. In addition, it has been very difficult finding skilled operators for construction equipment lately. In this regard, automatic or robotic construction equipment has been actively studied recently. In order to provide automatic or robotic construction equipment, the operational range of each working member of construction equipment should be automatically checked and controlled. For example, in power excavators, it is required to automatically check the displacement of the cylinder actuators corresponding to the boom, arm and bucket of the working members while performing the desired operations.
In order to automatically sense the displacement of typical cylinder actuators of the power excavators, the angles of joints between the working members of the power excavator may be sensed by positional sensors. Alternatively, the cylinder actuators may be provided with ultrasonic oscillators for checking the ultrasonic signals reflected by the tips of the actuating rods, thereby sensing the displacement of the cylinder actuators. The above typical method using either the positional sensors or ultrasonic oscillators are labelled "indirect sensing methods." As a further alternative, the displacement of the cylinder actuators may be sensed through a direct sensing method using magnetic graduations and sensors. In the above direct sensing method, the magnetic graduations of a magnetically transformed material are formed on each actuating rod. The sensor counts the number of magnetic graduations during the linear reciprocating motion of each piston relative to an associated cylinder, thereby sensing the displacement of the cylinder actuator.
The above method for sensing the cylinder's displacement by sensing the angle of the joints is particularly suitable to be used with either the boom or arm as the method is relatively cheaper and provides high operational precision. However, it is impossible to adapt the above method to the bucket which is typically operated under extremely coarse operating conditions. That is, as the bucket directly performs the operations while being brought into contact with the objects, it is impossible to use the above method for sensing the bucket cylinder's displacement. In this regard, the bucket cylinder's displacement has been sensed by the above-mentioned direct sensing method using magnetic graduations and sensors.
FIG. 1 shows the construction of a cylinder actuator whose displacement is sensed by the above-mentioned direct sensing method. In the above cylinder actuator, a piston 12 is received in a hydraulic cylinder 11 such that the piston 12 can linearly reciprocate in the cylinder 11 due to oil pressure. Axially extending from one side of the piston 12 so as to penetrate one end of the cylinder 11 is an actuating rod 13. A plurality of above-described magnetic graduations are formed on the surface of the actuating rod 13 at regular intervals. Spaced apart from the rod 13 by a predetermined distance A is a sensing means 14 which counts the number of magnetic graduations of the rod 13 during the reciprocating motions of the piston 12 in the cylinder 11, thereby sensing the stroke of the piston 12 in the cylinder 11.
An enlarged view of the sensing means 14 and the rod 13 in FIG. 1 is shown in FIG. 1A. A plurality of grooves, or graduations, 40 are formed directly on the rod 13 of the cylinder at constant intervals and are coated in chrome into the same external shape as the normal rod. In a cylinder head 15 of FIG. 1, a permanent magnet 49 and at least one stroke sensor 45 are mounted in a sensor holder 42. The permanent magnet 49 applies a magnetic field to the chrome layer 41 of the above groove 40 on the cylinder rod 13, thereby inducing its magnetic bias. The stroke sensor 45 is attached to a circuit board 44 for holding the sensor 45 and detects the magnetized degree of the chrome layer 41 in the groove 40. To put it more concretely, the output signal of the sensor has a sinusoidal form in general and can detect the displacement of the rod stroke of the cylinder by counting the sinusoidal output.
When the sensing means 14 is placed apart from the actuating rod 13 by a predetermined distance as described above, the sensing means 14 may fail to count the number of the magnetic graduations on the rod 13 accurately during the reciprocating motions of the piston 12 in the cylinder 11. In such case the sensing means 14 cannot detect the stroke of the piston 12 reciprocating in the cylinder 11 precisely, thereby resulting in the detected stroke of the piston 12 being different from the real stroke. On the other hand, when the sensing means 14 comes into contact with the actuating rod 13 in order to rectify the above problem, the sensing means 14 detects the stroke of the piston 12. However, in this case, the sensing means 14 may break because of the frictional heat generated from the actuating rod 13 during the reciprocating motion of the actuating rod 13.
In an effort to solve the above problems. Japanese U.M. Laid-open Publication No. Sho. 63-97804 discloses a sensor whose end portion, coming into contact with the reciprocating rod, is coated with a metal for improving the operational precision of the stroke sensing operation. However, the above Japanese sensor has a problem in that it is very difficult to coat metal on the nonmetal sensor. In addition, the frictional heat generated during the reciprocating motion of the actuating rod is directly transmitted to the above sensor, thereby causing the sensor to break and needing to be frequently changed with a new one.
On the other hand, Japanese U.M. Laid-open Publication No. Sho. 63-58707 discloses a somewhat improved structure for holding a sensor. In the above Japanese structure, the sensor 22 is held by a sensor holder 21 as shown in the accompanying drawing, FIG. 2. In addition, Japanese U.M. Laid-open Publication No. Heisei. 4-77007 discloses a sensor 31 whose top is provided with a slit 32 and which is mounted in the sensor holder 34 as shown in FIG. 3. The position of the above sensor 31 can be adjusted by a pin 33.